The invention concerns an all-optical converter for converting an intensity-modulated optical signal into an optical signal modulated to the DPSK format. It finds an application in the field of optical transmissions.
Intensity modulation by On/Off Keying or OOK is known. In this case, a pulse represents a “1” and an absence of pulse represents a “0”.
Differential phase shift modulation (Differential Phase Shift Keying or DPSK) is known. In the DPSK format, the information is transported in the phase difference between two adjacent pulses.
Transmission to the DPSK format is advantageous since the sensitivity is better. In addition in optical mode, it is easy to analyse the phase difference between two signals.
There currently exist, in the field of optical transmissions, converters that convert an amplitude-coded binary electrical signal into an optical signal coded to the DPSK format. These converters combine electronic elements and optical elements.
The electronic part of the converter comprises an exclusive-OR (XOR) electronic logic gate. The electronic signal that represents the information arrives modulated in amplitude at the first input of the logic gate. The output of the logic gate is connected to the second input of the logic gate with a delay of one bit time with respect to the first input.
In a particular case, an optical signal serving as a carrier is sent into an optoelectronic modulator, for example of the Mach-Zender type. This modulator is controlled by the electrical signal issuing from the output of the logic gate and effects a modulation so that a state of the electrical signal equal to “1” is associated with a phase difference of .pi. of the optical signal.
These converters, which combine electronics and optics, are very expensive for frequencies above 20 GHz and are non-existent for frequencies above 40 GHz.
The problem is therefore to produce an all-optical converter for converting an intensity-modulated optical signal into an optical signal modulated to the DPSK format.
The invention is not limited to certain frequencies and can apply to the entire spectrum.
Hereinafter 2 to 2 or 50:50 couplers will be referred to as 3 dB couplers.
Hereinafter it will be said that an output of a component is not connected if the signal that is delivered on this output is entirely absorbed, not interfering with the signals upstream or downstream. It will be said that an input of a component is not supplied if the amplitude of the signal that it receives is zero.
One object of the present invention is to propose a converter that does not have the drawbacks of the prior art.